1. Field of the Invention
The present invention relates to the art of electric arc welding and improved methods of welding two sides of a joint simultaneously.
2. Background
Submerged arc welding is a welding method where the arc creating the weld is shielded from atmospheric conditions by being submerged in a blanket of granular flux. In addition, the flux becomes molten during the welding process and provides a path for the current to travel to the metal workpiece (or “base”) being welded. Submerged arc welding can be performed with a DC or AC power source and typically uses high output currents. Where AC power sources are used, square wave sources are often employed. Using AC square waves rather than sinusoidal waves reduces the likelihood of having to re-strike the electrode to create an arc again.
Making welds on two sides of a piece of base metal can be difficult and prone to more defects and greater distortion than normal. In particular, welding both sides of a base for butt welds and fillet welds is complex for a number of reasons. FIGS. 1A-C show two plates of base metal joined with a butt weld. FIGS. 2A-C show two plates of base metal joined with a fillet weld. The edges to be welded are often welded flat as shown in FIGS. 1A and 2A or ground to a beveled edge as shown in FIGS. 1B and 2B. An example of a completed butt weld is shown in FIG. 1C and a completed fillet weld in FIG. 2C. Butt welds and fillet welds often require welding on two sides as shown in FIGS. 1A-C and 2A-C. These two-sided welds have been created by welding one side at a time or welding both sides simultaneously.
One method of welding two sides of a base metal is to weld both sides with two DC electrodes at the same time where both electrodes are either DC positive or DC negative. However, placing two DC electrodes close together often leads to arc blow. Arc blow occurs when a magnetic field interferes with the arc, causing it to wander as the weld is made. Arc welding often employs high currents that create strong magnetic fields near the arc and ground currents. These magnetic fields can push the arc around and create a wandering weld bead rather than a tight, controlled weld bead. Logically, arc blow presents a significant problem for a two-sided weld using a DC power source because the two arcs are close enough that their magnetic fields can interfere with each other. This has been solved by using a “leading” electrode and a “lagging” electrode, where the two electrodes are separated by usually at least four feet. That is, the two opposing electrodes travel along the weld joint at the same rate and in the same direction, but the leading electrode is positioned in front of the lagging electrode by at least four feet. The electrodes are separated to prevent arc blow and held at the same DC potential to force the current to flow through the base to the common node of the circuit.
This leading-lagging DC method, however, is prone to defects. For example, the weld pool from the leading electrode begins to cool and can start to solidify by the time the lagging electrode reaches it. This can introduce cracks in the weld bead. In addition, the separation between the electrodes and weld pools creates an uneven distribution of heat as the weld is created and the metal begins to solidify. This uneven heat distribution can cause distortion because the weld solidifies unevenly or can bend under the weight of the heavy metal base. Thus, while this solution provides a relatively quick weld, it is prone to defects.
The most common method with butt welds involves welding one side first, performing a back gouging step, and then welding the second side. This method can be performed using either DC or AC power sources, but is customarily performed with DC power supplies. The weld starts by affixing a backing to the location of the weld, which is often a copper plate (FIGS. 3A and 4A). Next, the first side of the base is welded—often with deep penetration and a high deposition rate depositing a significant amount of weld material (FIG. 3B). Many times, the butt weld is prepared by beveling the first side of the base, which then usually requires multiple passes to fill the open space (FIGS. 4A and 4B). After welding the first side, the backing is removed and the second side is “back gouged”. Back gouging cuts a bevel in the second side and removes a mix of metal from the base and some material deposited during the first weld (FIGS. 3C and 4C). After the second side is back gouged, it can finally be welded (FIGS. 3D and 4D). This solution provides a strong weld that is relatively free of defects, but it requires significant material and is a slow process. Currently, using DC positive with back gouging is the method most commonly used for butt welds in the 2G position.
A solution is needed to weld both sides of a base simultaneously and create a strong weld that is not prone to cracking or distortion. Such a solution would save welders the significant extra costs in labor and equipment currently caused by slower processes.